1. Field of the Invention
The present invention relates to a transparent, electrically conductive layer, to a process for producing the layer and to its use.
2. Description of the Background
Transparent layers with high ohmic conductivity have areal resistances of at most 1000 Ω/□ and a transmission of over 70% and are required in all modern displays, for example in LCDs, plasma displays, OLEDs, and, for example, also in organic solar cells, in order to be able to utilize the electrically currents excited by the photovoltaic effect with low losses.
A mechanically stable layer is understood hereinafter to mean a layer which has resistance to stress by scratching, sharp-edged objects or materials, characterized, for example, by the pencil hardness according to DIN EN 13523-4:2001.
Areal resistance is understood hereinafter to mean the ohmic resistance which is obtained on a coating with a uniform coating thickness when a square region of any size is contacted on two opposite edges and the current is measured as a function of the (direct-current) voltage. The areal resistance is measured in Ω and indicated by Ω/□. The areal resistance can also be determined by other methods, for example four-point measurement.
Specific resistance is understood hereinafter to mean the ohmic resistance which is obtained by multiplying the areal resistance by the layer thickness [in cm] and constitutes a measure of the ohmic properties of the conductive material itself. The specific resistance is reported in Ω·cm.
Transmission is understood hereinafter to mean the transmission of a transparent body for light of wavelength 550 nm. The transmission of a coated glass is reported in relation to the transmission of the same uncoated glass in percentages.
Hereinafter, transparent conductive substances are understood to mean compounds from which a transparent conductive film, abbreviated to “TCF”, can be produced. There has long been a search for a process which allows TCFs to be applied to glass or plastic surfaces in an inexpensive coating or printing process in order thus to be able to dispense with the technically complicated vacuum processes, for example sputtering, CVD or PVD, for the production of transparent conductive layers.
A series of patent applications describes the use of soluble metal compounds for the production of conductive transparent layers by means of coating or printing techniques. WO 98/49112 describes, in particular, the use of indium and tin compounds, and also antimony and tin compounds, which can be converted by pyrolysis or hydrolysis to indium tin oxide, hereinafter abbreviated to “ITO”. The precursor compounds can be pyrolysed by heating in an oven to over 500° C. or by laser irradiation, described in WO 95/29501. The precursor compounds used are indium octanoate and tin octanoate (JP 54009792), formates (EP 192009), chlorides (EP 148608), acetylacetonates (JP 61009467), nitrates (JP 02126511) and also organometallic compounds such as dibutyltin dioctanoate (JP 02192616) and trimethyl- or triethylindium, and also tetramethyl- or tetraethyltin (JP 6175144). In JP 6175144, the precursor compounds are decomposed by means of UV radiation and converted to ITO.
With the technical approach of hydrolysis, frequently referred to as sol-gel coating, conductive transparent layers with thicknesses of 100 to 500 and areal resistances of 200 to 1500 Ω/□ have been obtained. An exception is formed by EP 0192009, in which a layer of ITO with an areal resistance between 7.5 Ω/□ and 35 Ω/□ with a layer thickness between 90 nm and 300 nm is described by flame pyrolysis of a mixture of indium formate and dibutyltin oxide. However, this process has the disadvantage of an unsatisfactorily low transmission of the layer, which is between 79% and 82%. The specific resistances of the layers from these processes are typically a few 10−3 Ω·cm and can even be reduced down to 2·10−4 Ω·cm for very thin layers, as can be discerned from EP 0192009. These layers thus already have a conductivity typical of sputtered ITO layers. Experiments have shown that a greater value of in some cases over 90% transmission with areal resistance below 100 Ω/□ is obtained when a plurality of layers are printed one on top of another. However, this is much more complicated from a technical point of view and is therefore too expensive for commercial applications.
An alternative approach to the construction of highly conductive transparent layers with an areal resistance below 1000 Ω/□ in a coating or printing process consists in the use, for example, of ITO or ATO (antimony tin oxide) nanoparticles whose mean sizes are below 100 nm and are therefore significantly smaller than the wavelengths of visible light. These nanoparticles afford layers of high transmission of at least 90%, measured at a light wavelength of 550 nm (JP 2001279137, U.S. Pat. No. 5,662,962).
Instead of spherical nanoparticles, it is also possible to use fine, needlelike particles, described in U.S. Pat. No. 6,511,614. In the case of suitable production, the specific resistance within the particles is only a few 10−4 Ω·cm. The macroscopic areal resistance depends upon the contact of the particles to one another, the so-called percolation, and the conductivity of the medium between the particles. Since an nonconductive organic binder which enables a certain unspecified mechanical stability of the layer is used in U.S. Pat. No. 6,511,614, the specific resistance at over 0.1 Ω·cm is much too high to obtain highly conductive layers.
Particulate layers can be realized in layer thicknesses up to well over 1 μm. For this purpose, virtually all common coating and printing techniques are suitable, provided that the nanoparticles are well dispersed. The layers obtained by the process described in WO 03/004571 are, after the application and the evaporation of the solvent, consolidated by sintering processes. Energy required for this purpose is introduced by laser radiation or in a thermal manner. The layers obtained in this way are, however, highly porous. The porosity cannot be eliminated even by treatment at temperatures between 500° C. and 800° C. The specific resistance at 10−2 Ω·cm is therefore significantly above the values of the other above mentioned processes. An areal resistance below 100 Ω/□, which is desirable for highly conductive layers, therefore necessitates layer thicknesses above 1 μm. The use of such great layer thicknesses in modern displays is, however, technically disadvantageous and economically unviable. A further disadvantage of particulate layers is the low mechanical stability which, as a result of the sintering of the particles with one another and with the support material, is so weak that the layers can be wiped readily off the carrier. Binder is therefore additionally used. Binders in turn bring about the increase in the areal resistance.
There is one possibility of using conductive binders in order to increase both mechanical stability and electrical conductivity. In the simplest case, it is possible for this purpose to use conductive polymers. Since the common polymers are, however, p-conductive, while most of and the best conductive metal oxides are n-conductive, these materials are generally incompatible.
Another approach consists in using the TCOs (“transparent conductive oxides”) as binders. One embodiment of the use of precipitated metal oxides as binders between metal oxide nanoparticles in a sol-gel approach is described by JP 05314820. The formulation disclosed in JP 05314820 consists of indium oxide and tin oxide nanoparticles, and also hydrolysable indium and tin salts, in a solvent. In this case, the portion by mass of the particles of 2 g is significantly smaller than that of the metal salts, of which 45 g are used. The formulation is applied to a substrate, dried and at the same time hydrolysed, and calcined at 500° C. The layer thicknesses thus achieved are less than 100 nm, and areal resistances of at least 430 Ω/□ are realized. These values are too high for applications in displays or photovoltaic components. Obviously, several layers have to be applied, dried and calcined in succession in order to bring about lower areal resistances. One variation to this approach is described in DE 19754664. In this approach, in a first working step, conductive transparent layers of metal oxide particles, for example ITO or ATO, are applied in a solvent which is dried. A sol-gel coating which comprises oxidation-resistant metal particles or salts thereof, which are intercalated into the TCO layer, is applied thereto. The resulting layer has a very good mechanical stability, pencil hardness 8H, but areal resistances over 1000 Ω/□.
An essential common feature to all processes for producing transparent conductive layers based on TCO is the thermal treatment of the layer or the sintering of the particles. It is only this step that leads to a continuous layer which is mechanically stable and has a high transparency and at the same time high electrical conductivity. The state of the art is the heating of the layer on the substrate in an oven. However, the thermal behaviour of the substrate also has to be taken into account in the heating. For example, thermal expansion, deformations and changes on the substrate surface during heating, or the formation of mechanical stresses which have to be kept within limits if the TCO layer is not to be affected adversely, necessitate time-consuming and hence costly control of the profile of temperature with time. For the same reasons, the temperature which can be established in the thermal treatment is limited. Some substrates, for example plastics, must not be heated to such a high degree as would be required to achieve optimal electrical conductivity and transparency of the TCO layer.
DE 199 40 458 A1 discloses a process for thermally modifying electrically at least semiconductive coating materials which are subjected to a high-frequency electromagnetic field. The thermal action of the electromagnetic field is based on convection currents induced in the material. The frequency of the electromagnetic field which is applied to the coating material to be modified thermally in one step is within the range from a few kilohertz up to a maximum of a few megahertz, preferably in the range from 100 to 500 kHz.
Current transparent ITO contacts for touchscreen applications, flat screens based on LCD or OLED modules or illumination modules based on OLEDs or ELs and for solar cells are structured by standard lithography steps. To this end, ITO (indium tin oxide) is typically deposited under reduced pressure (for example sputtering technique) over a large area on a substrate (for example glass). Thereafter, by means of illumination of an applied light-sensitive coating by masking and subsequent etching-away of the ITO, the desired structure is obtained.
One disadvantage is that a certain proportion of the material applied over a large area, as a result of the subsequent etching, necessarily cannot be utilized viably. In view of the demand for ITO and the rise in the price of indium, there is potential here for material saving and hence cost saving, and, to some extent, the lower the effective areal covering of the ITO on the substrate is, the greater this potential.
Alternatively, the desired ITO tracks can also be deposited by means of shadow-masking, but what is known as under-sputtering often occurs here as a result of scattering effects, which leads to indistinct edges and hence to imprecisely defined structure edges. The problem of material waste outlined above likewise occurs here.
If a change in the layout is required, new masks have to be made up in both cases, i.e. the structuring is always tied to the mask design. This is also accompanied by a restriction in the variability of the substrate size.
Further known deposition processes of ITO are: PVD, CVD, evaporation, spray pyrolysis, pulsed laser ablation, ion beam deposition, among others. In addition, processes based on wet-chemical processing are in development, although the layer resistance is only of minor significance. The sol-gel technique (see, for example, Aergerter et al., Journal of Sol-Gel Science and Technology 27, p. 81, 2003) is particularly highly established here.
The conductivity is generally scaled to the layer thickness of the contact. However, the absorption of light also rises with increasing layer thickness. Commercially available ITO conductor tracks have been optimized for both aspects. In the case of very good quality of a 100 nm-thick ITO layer, resistances of 20-50 ohm/square are achieved. The transparency is 90-95% in the visible range. The layers are usually deposited by means of a magnetron sputtering technique under elevated temperatures (>200° C.).
Transparent base contacts (e.g. ITO) which are applied by means of sputtering processes exhibit, in spite of their low roughness of about 1 nm RMS, typically local height differences (spikes) of several nanometers (>10 nm). The reasons for this under some circumstances lie in the order of magnitude of the thickness of the organic layers. In such cases, the probability of short circuits, local current density peaks and elevated leakage currents rises, which usually results in a short lifetime of the OLED component. If necessary, the ITO contact is subsequently subjected to a polishing step, which is, however, time-consuming and costly.
It was an object of the present invention to find novel transparent, electrically conductive layers. An important aspect is the simple and material-saving incorporation of the production of such a layer into the industrial manufacturing process, especially of light-emitting components and displays.
Such a layer should, as well as ensuring the physical properties of the contact, in particular good conductivity and high transparency and low roughness and low probability of short circuits, satisfy the following points in particular:
Optimal material yield, very few process steps and avoidance of complicated processing under reduced pressure in the production, avoidance of structuring masks, variability in the layout and design of the structures, variability of the substrate size and simple scalability for large areas.